This invention relates to a flash exposure photocopying machine and, more particularly, to an automated system for controlling photoreceptor exposure in response to detection of original documents of varying color backgrounds.
In electrophotographic devices, such as a xerographic copier or printer, a photoconductive surface is charged to a substantially uniform potential. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced, forming an electrostatic latent image at the photoconductive surface corresponding to the informational areas contained within the original document. The electrostatic latent image is subsequently developed by bringing a developer mixture into contact therewith. The developed image is then transferred to an output copy sheet and the powder image on the output sheet is heated to permanently affix the image to the sheet.
The documents that are copied may vary considerably in the actinic density of the paper substrate. White documents may have absolute densities that range from 0.04 to 0.20 depending on factors such as quality and age. Documents with colored backgrounds may have phototopic densities as high as 0.50. Copiers without compensation tend to produce unacceptable background when copying colored background documents. Various compensating techniques, know as background suppression or background stabilization, are implemented to reduce the undesirable side effect of solid-area sensitivity. For example, it is known to provide manual background stabilization in the form of a range of copy-lighter/copy-darker settings on the control panel of a copier. The high background from a colored original is suppressed by selecting a copy/lighter mode which typically increases exposure, or developer bias, or a combination of the two. This technique has the disadvantage of being a hit or miss technique and causes considerable delays in the copying operation until suitable settings are found for the particular document.
It is also known to provide various forms of automatic background stabilization. For example, the Minolta 350 copier uses a passive developer-bias control that increases bias when the average image potential increases. The 3M "Sensitron" copier increases exposure when the average reflectance across the process decreases. This technique compensates for variations in substrate density for low-area-coverage documents but, undesirably, also compensate for areas of high image density, mistaking the image density for background density. Another example of an automatic background stabilization technique is that found in the Canon NP-270F copier. For this technique, the scanning system performs a prescan of the document, sensing the background image potential with a built-in electrometer. Exposure is then adjusted prior to the actual exposure scan. This technique is also disclosed in U.S. Pat. No. 4,200,391. The disadvantage in this type of correction is undesirable loss of productivity due to the extra time required for the process.
Other background sensing circuits for scan-type systems are disclosed in U.S. Pat. Nos. 4,153,364 and 4,372,674. In U.S. Pat. No. 4,153,364, a scanning linear photosensor array detects maximum and mdinimum output levels of a light image incident thereon and uses the information to control the illumination intensity of the document and the developing bias voltage. U.S. Pat. No. 4,372,674 discloses a copying machine having detectors for the background color and density of an original document and having means for generating a bias voltage based on these measurements.
Another technique to automatically stabilize background is to sense the charge level of an exposed image by an electrometer positioned between the exposure and developer stations and to adjust the developer bias to compensate for changing background levels. Representative of this technique is the disclosure in U.S. Pat. No. 3,877,413 and co-pending application 901,990, assigned to the same assignee as the present invention.
Compensation techniques for flash exposure systems have tended to be costly and/or limited to function. U.S. Pat. No. 3,877,413 cited above discloses a flash system, but requires that the developer bias be changed. It would be desirable for a flash type system to regulate the lamp exposure so as to to provide the exact component of exposure required by the particular background of the document being copied. U.S. Pat. No. 4,334,767 discloses such a system in which a lens/sensor placed in the floor of a flash lamp housing generates a real time signal which is integrated and used to control the flash lamp exposure time. This mechanism is suitable for some systems, but provides only a rough approximation of the document background since the sensor output includes a mixture of foreground and background information.
It would be desirable to utilize an exposure control system with improved discrimination between background and foreground density of the original. The present invention is directed toward an exposure control system which utilizes a photosensor with a plurality of photosites, each photosite sensitive to the background of a discrete portion of the document being copied. Photocurrent outputs from the photosites are integrated to determine the lowest background density. A representative signal of the density is used to quench the flash at the point of optimum exposure. More particularly, the invention relates to a flash exposure document imaging system wherein a document is flash-illuminated and an image projected by a lens within the housing onto the surface of a photoreceptor and includes an automatic exposure control mechanism comprising: a sensor array having a plurality of sensor elements therein, said array located within said housing at a location such that an image of the document is projected onto said array coincident with said flash illumination, circuit means for integrating photocurrents produced by each of said plurality of photosensors, comparator means for receiving the integrated outputs and for comparing the outputs with established reference levels, and circuit means for quenching said flash means as a function of the comparator output.